The present invention relates to safety interlock and protection circuits used in a DC-AC inverter motor controller driving a Permanent Magnet Synchronous Motor (PMSM) and particularly to a safety circuit implemented in a gate driver integrated circuit (IC) designed to drive the PMSM.
A PMSM has become an important power source for energy efficient appliances including air conditioners, refrigerators, washers, etc. The PMSM has higher power density and higher torque-per-amp than an induction motor. Induction motors have been commonly used in the past in appliance applications, e.g., washing machine applications. PMSMs have been used in more modern washing machines and have become an important power source for energy efficient appliances such as washing machines but also including air conditioners and refrigerators.
PMSMs are often driven by a DC-AC inverter circuit fed from an AC supply. That is, the AC main supply is first rectified/converted to a DC voltage, which is maintained across a DC bus. The DC bus voltage is supplied to the inverter to be converted to AC for driving each of the phases of the PMSM. There are typically three phases of the PMSM. Unlike the induction motor, when the PMSM is used with AC inverter circuits, it requires additional protections to guarantee the safety of the inverter. This is so especially when the PMSM is driven in the field weakening range. Driving a PMSM in the field weakening range is often mandatory, particularly in the automatic washing machine application in washers/spin extractors with a high-speed drum operating in the spin mode.
A micro-controller and/or a digital signal processor is a typical component for controlling the inverter. These control the inverter power switches, which are typically IGBTs or FETs, to apply the desired drive voltages to the PMSM, typically pulse width modulated (PWM) signals. Gate driver ICs are used between the controller device and the power switches for providing variable frequency/voltage to the PMSM, thus controlling the torque and speed of the PMSM. The gate driver is a critical element of the circuit. When the switch gates are not driven ON, the power switches remain in an OFF state, while the PMSM's energy may re-circulate through freewheeling diodes anti-parallel to the power switches, thus re-charging a bulk capacitor. In low cost applications, no provisions are made to return this energy back to the power line. All the energy is absorbed by the bulk capacitor.
The magnetic field is always present in the PMSM, as it is produced by the permanent magnets. This is different from an induction motor, in which the magnetic field has to be generated by proper control. In the event of a controller failure, i.e., due to a power line failure, brown out or even unplugging of the power cord, or a loss of the gate driver auxiliary power supply, the PMSM can exhibit two potentially dangerous conditions when operated in the field weakening region. The first is when the PMSM spins at high speed when coasting while slowing down. This condition creates a safety hazard and may result in bodily injury. The second condition is that a PMSM may generate over-voltage on the DC bus when the controller disengages from energizing the PMSM as a result of controller failure.
When in field weakening operation, the speed of the PMSM reaches very high values. During that time the counter Electromotive Force (EMF), known as Back EMF, generated by the motor when it spins freely, can be much higher than that of the rated value of the bulk capacitor. In a case of loss of control, such Back EMF can easily lead to the destruction of an inverter circuit and capacitor in addition to presenting a fire hazard.
The motor generated voltage can be particularly high when the motor is operated under a field weakening control to achieve a very high spin operation. If a failure occurs at that time, the generated voltage is very high. The voltage is proportional to the product of motor speed and flux generated by the PMSM as the rotor moves with respect to the windings. In the field weakening mode, if the controller fails and is unable to control the motor, the weakened flux changes to the full amount of flux (due to loss of flux weakening) while the motor speed may be reaching more than three or four times its nominal operating speed depending on the spin mode speed, resulting in high Back EMF.
If no action is taken during such a controller failure during field weakening operation, the DC bus voltage may reach approximately three to four times the nominal DC bus voltage. This will result in damage to the power devices and high voltage ICs in the system. The DC bus capacitor can also be damaged. Once damage occurs in the power system, it may no longer be possible to provide a safety door lock/braking mechanism since any circuits on the IC board will likely be damaged as well or be rendered inoperative.
Accordingly, it is desirable to manage, at the gate driver IC level, the causes for such lack of control during field weakening operation that could lead to generation of this high back EMF at high speed operation and prevent it. Thus the present invention has as an object to prevent a loss or brown out of the power line mains source or a loss of the gate driver auxiliary power supply from leading to a loss of control of the PMSM during high speed operation and to allow for rapid reduction of the motor and drum speed without generating high Back EMF that could damage the system components.